VGCF has excellent material properties of a unique onion-ring micro-structure, a high aspect ratio, a high thermal-conductive coefficient, a low thermal-expansion coefficient, high strength, high elasticity and high corrosion resistance. In addition, carbon fibers made by the vapor-growing method can have the structure similar to the single-crystal graphite structure, thereby forming excellent multi-wall carbon tubes having excellent electrical conductivity, wherein the thermal conductivity thereof is better than that of the thermally-conductive material such as copper or aluminum. The success of VGCF study has added quite an important product to the carbon fiber industry in which OPCF (Organic Precursor Carbon Fibers) such as PAN, Pitch carbon fibers have been the major products for quite a long time.
The VGCF production process mainly uses low boiling hydrocarbon compounds as raw material (carbon source) having pyrolysis reaction in reductive carrier gas (H2) atmosphere, thus directly forming VGCF via the special catalysis of transition metals such as iron, nickel or cobalt in nano-particles thereof as nucleation, wherein the reaction temperature is between 800° C. and 1300° C. The VGCF fabrication process has the advantage that the fabrication skill is simple and does not need to perform the steps of spinning, pre-oxidation, carbonization, etc. required in the OPCF fabrication process, so that the VGCF fabrication process can form carbon fibers directly from cheap low-boiling hydrocarbon material via pyrolysis and catalysis.
Referring to FIG. 1A and FIG. 1B, FIG. 1A and FIG. 1B are schematic diagrams showing conventional VGCF reaction apparatuses respectively, wherein a conventional VGCF reaction apparatus is composed of tubular reactor and a heater 50, and the tubular reactor can be formed solely from an outer tube 40 (such as shown in FIG. 1A) or from an inner tube 30 inserted into the outer tube 40 (such as shown in FIG. 1B).
Such as the tubular reactor shown in FIG. 1A, raw material gas enters the outer tube 40 via an guide tube 10 mounted on one end of the outer tube 40, and the heat generated by the heater 50 is passes through the outer tube 40 to the gas mixture of the raw material gas and carrier gas for increasing the temperature of the gas mixture, thereby pyrolyzing the raw material gas to form carbon fibers. Thereafter, the carbon fibers generated fall into a collection bin 60. However, in the aforementioned process, the temperature near the tube wall is quite higher than the central area of the reaction tube and the situation becomes worse when the reactor diameters increase, so that aforementioned process is merely suitable for use in the tubular reactors with small diameters and is not suitable for mass production. Meanwhile, there are carbon fibers frequently attached to the tube wall, thus lowering productivity, and the reaction often needs stopping for cleaning, thus disadvantaging continuous production.
Such as the tubular reactor shown in FIG. 1B, raw material gas enters the outer tube 40 via a guide tube 10, and carrier gas (H2) enters the outer tube 40 via a gas inlet 20 mounted on one end of the outer tube 40, and inert gas enters from the bottom of the outer tube 40 as guide gas. After mixing, the raw material gas, and the carrier gas enter the inner tube 30, wherein the heat generated by the heater 50 is passed through the outer tube 40 to the inner tube 30 and then to the gas mixture of the raw material gas, the carrier gas and inert gas for increasing the temperature of the gas mixture, thereby enabling the raw material gas to be pyrolyzed to form carbon fibers. Thereafter, the carbon fibers generated fall into a collection bin 60. Generally, the inert gas passing between the inner tube 30 and the outer tube 40 is used as guide gas for reinforcing the heat transfer efficiency between the inner tube 30 and the outer tube 40. However, since the thermal conductive coefficient of the guide gas is not large, the heating efficiency of the heater 50 on the gas mixture is not good, and the energy provided by the heater 50 thus cannot be well utilized, and the VGCF formed from carbon source under inert atmosphere is inferior to that formed under pure reductive atmosphere (H2). Further, there are carbon fibers frequently attached on the tube wall of the inner tube 30 in the conventional tubular reactor, thus lowering productivity and disadvantaging continuous production due to frequent tube wall cleaning.
On the other hand, a conventional continuous production system for producing VGCF is mainly composed of a gas-supplying apparatus, a tubular reactor and a collection bin. After the carbon fibers generated fall into a collection bin and fill up the collection bin, the reaction has to be first stopped, and then a gas-swapping step is performed for replacing the carrier gas (H2) in the collection bin with inert gas (such as nitrogen) so as to prevent carrier gas from resulting in explosion. Thereafter, the collection bin is moved out and replaced with another empty collection bin. Then, after the air in the empty collection bin is expelled and replaced with inert gas, the carrier gas is introduced into the reaction system for starting another reaction cycle. Therefore, the conventional continuous production system has the following disadvantages. Besides replacing the collection bin and the atmosphere therein, a lot of manpower have to be consumed and further the reaction system has to be stopped for quite a period of time, thus resulting production interruption and lower productivity; and there is no carrier-gas recycling system, not only resulting in pollution but also increasing production cost.
Hence, there is a need to develop a reaction system for producing VGCF in a continuous manner, thereby effectively utilizing the energy provided by the heater; preventing carbon fibers from attaching to the tube wall of the inner tube; continuously collecting products (VGCF) without stopping the reaction apparatus; and effectively recycling carrier gas, thus increasing productivity; easily cleaning the reaction tubes; preventing pollution and workforce waste; and lowering production cost.